1. Field of the Invention
The present invention relates to an exposure apparatus adapted for use in manufacture of semiconductor devices, substrates for liquid crystal display panels or the like, and more particularly to an exposure apparatus for effecting so-called image area synthesis or composition for forming a pattern of a large area by partially superposing patterns of a unit area on a photosensitive substrate.
2. Related Background Art
In such exposure apparatus, in order to meet the requirement for a size increase of the photosensitive substrate to be exposed, there is employed an image area synthesis method, in which the exposure area of the photosensitive substrate is divided into plural unit areas and exposures are executed in repeated manner, respectively corresponding to such unit areas to finally synthesize a desired pattern. In such image area synthesis method, in order to avoid pattern discontinuity at the boundary of the unit exposure areas, possibly resulting from the error in patterning on the reticle for pattern projection, optical aberrations in the projection optical system and alignment error of the stage for positioning the photosensitive substrate, exposures of the unit areas are executed with mutual overlapping by a small amount at the boundary. However, such overlapping of the exposure areas gives a doubled exposure to the overlapping area, possibly resulting in a variation in the pattern line width in the pattern junction area, depending on the properties of the photosensitive material employed. Also such image area synthesis may generate a step difference at the junction of patterns, due to the positional aberration between the adjacent exposure areas, eventually deteriorating the properties of the device to be produced. Furthermore, if such pattern, obtained by the image area synthesis method, is overlaid in plural layers, with respectively different exposure apparatus, the error in overlapping of the exposure areas fluctuates in different manners at the pattern junctions in different layers, due to the difference in the lens aberrations and in the alignment precision in different exposure apparatus. Particularly in case of an active matrix liquid crystal display device, such phenomenon leads to a discontinuous variation of the image contrast at the junction of the patterns, thus significantly deteriorating the quality of the display device.
For avoiding such drawback in the image area synthesis, the Japanese Patent Publication No. 63-49218 discloses a method of providing the reticle or a filter overlaid thereon, with light attenuating means for reducing the amount of transmitted light in a position corresponding to the pattern junction, thereby reducing the amount of exposure in the overlapping portion of the patterns substantially equal to that in other areas.
However, such known method is associated with the following drawbacks. In case such light attenuating property is given to the reticle itself, it increases the burden to the preparation of the reticle, for example because of the increased number of steps required for the preparation and the increased probability of pattern defect generation in the course of preparation. On the other hand, in case such property is given to the filter to be overlaid on the reticle, the maintenance of the reticle becomes more difficult as the attaching and detaching of such filter increases the probability of damage and contamination of the reticle. Also as the reticle is usually provided, on both sides thereof, with pellicles of a certain thickness in order to prevent deposition of dust or the like onto the reticle pattern, the filter and the reticle pattern are inevitably separated by the thickness of the pellicle at minimum, so that the ideal light attenuating characteristics are difficult to attain on the reticle pattern. Besides an exclusive filter has to be prepared for each reticle, and considerable work will be inevitable for the preparation and maintenance of such filters.